1. Field of the Invention
The present invention relates to an overcurrent protector for a power element which is operative to protect the power element, such as an IGBT, a power MOSFET or a bipolar transistor, against an overcurrent.
2. Description of the Background Art
FIG. 6 is a circuit diagram showing a conventional overcurrent protector for protecting a power element, inserted in a load circuit of an inverter or the like that is designed to carry out the velocity control of an induction motor, against an overcurrent or a short circuit.
Referring to FIG. 6, an IGBT or similar component 1 is used as a power element. A gate amplifier 2 is employed as control signal amplifier for driving the power element 1. A control circuit 3 is operative for outputting a control signal to the gate amplifier 2. A shunt resistor 4 is connected in series with the power element 1 for detecting an overcurrent, and an isolating circuit 5 is used for isolating and transmitting the potential difference of the shunt resistor 4 to the control circuit 3.
Generally, the main circuit for the power element 1 is electrically isolated from the control circuit 3 by the gate amplifier 2. To provide overcurrent protection for the load circuit, i.e. the main circuit, a main current Ic of the power element 1 is detected by the shunt resistor 4. The detected Ic current value is provided via the electrically isolating circuit 5, such as an insulation amplifier, as a detection signal to the control circuit 3. As a result, the power element 1 is shut off when the detection signal reaches a predetermined level.
FIG. 7 is an equivalent circuit existing at a time when an overcurrent, particularly a short circuit, occurs in the main circuit including the power element 1 shown in FIG. 6. FIGS. 8A-8C illustrate the operation of the circuit shown in FIG. 7. Referring to FIG. 7, a main circuit power supply 6 is connected in parallel with a power element 1, which has a parasitic capacity 7 on its collector and gate. A switch 8 is also shown in the circuit, for representing a short circuit on an equivalent basis.
In the circuit shown in FIG. 7, when the switch 8 is open, the voltage V.sub.CE between the collector and emitter of the power element 1 is 0V. If, in this state, a short circuit occurs, i.e. the switch 8 is closed, as shown in FIG. 8A, a large current Ic begins to flow in power element 1, as seen in FIG. 8B. Also, a step voltage which is developed from the DC voltage V.sub.DE of the main circuit power supply 6, is impressed across the collector and emitter of the power element 1. The current is seen to rise sharply, peak and then drop to a stable level, this response probably being due to circuit line inductance.
The parasitic capacity 7 existing in the power element 1 causes a voltage V.sub.GE between the gate and emitter of the power element 1 to rise .DELTA.V.sub.GE from a standard voltage V.sub.G1, as shown in FIG. 8C. As a result, V.sub.GE exceeds the voltage of the driving DC power supply in the gate amplifier 2, further increasing the collector current Ic.
FIG. 9 is a detailed drawing of the circuit of the gate amplifier 2, acting as the control signal amplifier shown in FIG. 7. In FIG. 9, a gate driving DC power supply 9 is connected in a circuit with a photocoupler 10, comprising an LED 10A and a phototransistor 10B for isolating and receiving a control signal from the control circuit 3. Also included in the circuit are resistors 11, 13 and 16, and transistors 12, 14 and 15. A diode 17 is inserted between the gate G of the power element 1 and the positive terminal of the gate driving DC power supply 9.
In order to suppress the rise of the gate voltage V.sub.GE of the power element 1 when an overcurrent flows in the power element 1, the diode 17 is connected in the conventional design between the gate terminal G of the power element 1 and the positive terminal of the gate driving power supply 9 in the gate amplifier 2 (positive terminal of V.sub.G), as shown in FIG. 9. The diode 17 is for clamping the gate voltage V.sub.GE of the power element 1 to V.sub.G1, thereby limiting the peak value of current Ic to a reduced value at the time of a short-circuit accident. For example, if V.sub.G, is 15 volts, then V.sub.GI could be changed from a value of 15 volts to a higher value (e.g., 18 volts) and Ic could flow at a higher level when a short circuit occurs. However, the presence of the diode will limit the voltage rise, and Ic is correspondingly limited. Specifically, although the voltage will tend to get higher due to a short circuit, the diode suppresses the gain and the power is reduced. However, when the power is reduced, the peak current is suppressed and the circuit will not detect the overcurrent. As a result, the period between the start of the short circuit to the damage of the power element 1 is increased, i.e. a short-circuit capacity is improved.
However, the conventional circuit has several problems. First, because the known overcurrent protector for the power element is constructed as described above, the shunt resistor inserted into the load circuit of the conventional embodiment shown in FIG. 6 gives off a substantial quantity of heat. Second, the isolating circuit 5 provided for overcurrent detection is relatively expensive, since the increased loss may require a large-sized unit size and involve higher costs. Further, the current detector employed in the embodiment shown in FIG. 6 is required to detect a short circuit and shut off the power element. This also results in an enlarged unit size and higher costs.